Recently, electronic devices have become rapidly portable and cordless, and there is an increased demand for secondary batteries that are small, light-weight, and have high energy density, as the power source for driving such devices. Also, in addition to secondary batteries for use in small consumer products, technology is quickly being deployed for large secondary batteries for use in, for example, power storage devices and electric-powered vehicles that are required to be durable and safe for a long period of time.
Among secondary batteries, non-aqueous electrolyte secondary batteries, particularly lithium secondary batteries, are highly expected as the power source for small consumer products, power storage devices, and electric-powered vehicles, due to their high voltage and high energy density.
While higher energy density is required of lithium secondary batteries, a material containing an element such as Si, Sn, or Ge capable of alloying with lithium (i.e., material such as an oxide or alloy of the element) is highly expected to be utilized as a negative electrode active material with a high theoretical capacity density. Particularly, Si and Si oxides are studied extensively due to their low cost.
However, regarding a material containing an element capable of alloying with lithium, its crystal structure changes during lithium absorption, which causes its volume to increase. If the volume of the negative electrode active material greatly expands during charge, electrical contact between the negative electrode active material and the negative electrode current collector becomes poor, which causes the charge/discharge cycle life to become shorter. Moreover, a material containing an element capable of alloying with lithium has a large irreversible capacity, and thus has a problem of causing low initial charge/discharge efficiency (ratio of initial discharge capacity to initial charge capacity).
Therefore, the following proposals have been made.
Patent Literatures 1 and 2 propose a negative electrode mainly composed of a silicon oxide and a pre-doping of lithium into the negative electrode, in view of improving the initial charge/discharge efficiency of the negative electrode. These references explain that this pre-doping of lithium enables inclusion of lithium equivalent to the irreversible capacity of the negative electrode, into the negative electrode active material in the battery, which causes the initial charge/discharge efficiency and the average discharge voltage to improve.
Patent Literature 3 proposes adding a silicon oxide to a negative electrode mainly composed of graphite and using a positive electrode containing a lithium nickel oxide which has a large irreversible capacity, for the purpose of suppressing increase in the negative electrode potential at the final stage of discharge.